The invention relates to a continuously operating press for the production of particle boards, fiber boards or similar wood boards and plastic boards.
In the continuously operating press according to German Offenlegungsschrift 40 17 791, the upper press/heating platen is vertically adjustable for the setting of the press nip and the lower press/heating platen is deformable by means of a plurality of cylinder-piston arrangements or hydraulic actuators disposed in rows longitudinal and transverse to the longitudinal axis of the press, a press ram comprising a plurality of individual beams, which are interconnected elastically and mutually adjustable vertically. By virtue of the elastic configuration of the press ram, in order to support and enable the control of the course of the steel belt during running of the continuously operating press in the entry direction to the left or right, a pressure or path profile can be briefly regulated in accordance with the adjustment strategy. This means that the press ram is transported briefly from its horizontal position into an oblique position, by means of external short-stroke pressure cylinders, and the course of the steel belt is then corrected, such that it is reset, by an adjustment of the driving and/or reversing drum axes.
In the prior art, in order to control the procedure, all continuously operating presses must accurately reproduce the process sequence, as known from the intermittent-operation press technology for the production of particle boards, MDF boards (Medium Density Fiber) or OS (Oriented Strand Boards) boards. The procedure for the pressing force influence upon the pressing stock and degasification time is essentially executed by longitudinal deformation along the pressing zone. In continuously operating presses, the spherical deformation transverse to the pressing zone is additionally necessary, i.e., at least one of the two press/heating platens must be deformable between flatness (primarily in the calibration area in the exit region of the pressing zone, i.e., the low-pressure region) and a convex geometry (primarily in the entry region of the pressing zone, in the high-pressure and in the medium-pressure region), dependent upon the board thickness of the pressing stock, the bulk density and the moisture content of the particle/fiber pressing stock, to be able to regulate the minimum possible pressing factor (the maximum possible steel-belt=production speed) where optimal physical process requirements are placed upon the pressing stock itself, such as transverse tensile strength and bending strength. It is necessary along the pressing zone, longitudinally and transversely, to be able to set different nip clearances between the upper and lower press/heating platen, so as to be precise with the following nip clearance differences: longitudinal deformation (Delta I) about 0 to 3 millimeters per meter and transverse deformation (Delta q) about 0 to 1 millimeter per meter.
According to the continuously operating presses according to German Patent Specification 31 33 817 and German Patent Specification 39 14 105, the spherical influence upon the pressing stock is effected two-dimensionally with the upper press/heating platen. The longitudinal deformation is effected by the hydraulic actuator rows in each press frame along the pressing zone. The deformation is system dependent, due to the slab having a counter-heating system, and its higher inherent stiffness. The transverse deformation is herein effected essentially by the counter-heating in the slab. The counter-heating temperatures can be set differently along the pressing zone. For example, in the front region, in order to give a stronger convex deformation, lower counter-heating temperatures are employed relative to the press/heating platen temperature. By contrast, in the exiting low-pressure region of the pressing zone, in order to obtain the necessary flatness tendency, the counter-heating temperatures are regulated higher. The convex bending deformation is additionally supported at each press frame by minimizing the hydraulic actuating forces of the outer cylinders of the actuator row. However, this is only possible to a limited degree, since the slab system possesses high inherent stiffness. By virtue of the fact that the entire system operates relatively sluggishly in terms of the adjustability in the direction transverse to the transport of the pressing stock, an on-line adjustment in an economic fashion is not possible in just a few seconds. In other words, a flying production changeover or rapid optimization of the start-up procedure, for which a continuously operating press was supposed to be designed, is either not at all possible or possible only within limits. Nevertheless, the spherical deformation along the pressing zone in the longitudinal and transverse directions can be carried out preventively at idle, prior to the pressing stock being driven into the continuously operating press.
In continuously operating presses having bottom-piston systems, as in German Offenlegungsschrift 21 57 746, German Offenlegungsschrift 25 45 366 and German Utility Model 75 25 935, the lower press/heating platen, as in the previously described top-piston system, is deformed two-dimensionally. The press/heating platen is of relatively thin and consequently very flexible construction. By means of multiple hydraulic bottom pistons, convex transverse deformations can be brought about by differently regulated force influences in these bottom pistons for each press frame. The longitudinal deformation for different compression or degasification states of the pressing stock along the pressing zone is realized by altered path positions (spindle adjustment of the servo valve). The triggering of an altered longitudinal or transverse deformation can be effected on-line and it is not carried out preventively at idle prior to feed-in of the pressing stock, since the press/heating platen is supported, lying only in non-positive engagement on the hydraulic plunger-bottom piston carpet. A further drawback is that, in this construction, the different press nip for the pressing stock between the upper and lower press/heating platen can be set only by the reactive resetting forces of the plastic pressing stock as the bending and buckling stiffness of the lower press/heating platen is simultaneously surmounted. This means that the technological production parameters for the spherical deformation cannot be exactly reproduced. Furthermore, upon each production changeover and re-start, a greater start-up wastage of the pressing stock is inevitable.